Photosystem II contains two symmetry related redox active tyrosine residues, Y D (D2-Tyrl6O) and Yz(DI-Tyrl6l). Yz is thought to serve as an electron transfer intermediate between the tetranuclear Mn cluster, where water oxidation occurs, and the photo-oxidized chlorophyl moiety P 680' In particular, Yz is thought to play a direct catalytic role in the water oxidation by abstracting a proton from water molecules ligated to the metal cluster active site. To probe the function of Yz in greater detail, we are investigating the hydrogen bonding state of Yz and the proximity of Yz to the metal center. It has been proposed that Yz may have hydrogen bonding interactions with both a histidine residue and a water molecule. The shift in low field electron g value, g,, from the free electron g-value observed at high frequency has been shown to be sensitive to hydrogen bonding at the phenyl oxygen of the tyrosyl radical. A decreased shift in g, is observed with increased hydrogen bonding. The high frequency (139.5 GHz) EPR spectrum of the Yz tyrosyl radical of photosystem II in Mn depleted samples displays a rhombic powder pattern with principal g values of g,=2.00760, 92 =2.00450, and 93 =2.00230. Splittings along the low field turning point are observed that may arise from different hydrogen bonding environments forYz radicals in these Mn depleted samples, in which some molecules retain two hydrogen bonds and others only one. The position and widt h of the g, edge for Yz is consistent with such a disordered hydrogen bonding environment. Studies are also underway to measure the exchange and dipolar couplings between the tyrosyl radical and the Mn cluster in intact photosystem 11 samples where the tyrosyl radical has been trapped by treatment with acetate or acrylate.